In recent years, as image sensors are made increasingly high-performance and compact, digital appliances furnished with an image input capability, such as cellular phones and portable information terminals provided with an imaging optical device, have been becoming more and more popular. And there have been increasing demand for further compactness and higher performance in imaging lens systems incorporated in imaging optical devices. Imaging lens systems for such applications conventionally have a three-element or four-element construction; today there are also proposed imaging lens systems having a five-element construction.
In general, aiming at higher performance by increasing the number of lens elements runs counter to seeking compactness, and it is technically challenging to achieve both. One such attempt is disclosed, for example, in Patent Document 1 listed below. Patent Document 1 discloses an optical system in which compactness is sought mainly by defining the focal lengths, center thicknesses, and Abbe numbers of a first and a second lens element. As another attempt, Patent Document 2 listed below discloses an optical system in which compactness is sought likewise by defining the radius of curvature etc. of a second lens element.
High-performance imaging optical devices are generally furnished with a function of so-called automatic focusing. Inconveniently, however, with conventional constructions, the following problems have been becoming increasingly apparent: since they adopt a focusing method involving the moving of three to five lens elements together, they require a large driving mechanism, making it unfeasible to make the lens unit as a whole compact; in a part where the driving portion is provided, dust is produced, affecting the image quality; the driving mechanism may have an eccentric error, degrading the image quality. Since, inherently, it is technically challenging to seek high performance and compactness simultaneously, no breakthrough is possible without a radical change to the conventional focusing method which involves the moving of a whole unit.
One such attempt is seen, for example, in Patent Document 3 listed below. Patent Document 1 discloses an optical system in which an attempt is made to make the driving mechanism compact by moving, mainly, a first lens element alone. As another attempt, Patent Document 4 listed below discloses an optical system in which focusing is performed by moving a second lens element alone out of four constituent lens elements.